This invention relates in general to thickness measurement of structures and, in particular, to a system for measuring film thickness through laser induced acoustic pulse-echo using non-contact interferometry.
Ellipsometry is a powerful technique for film thickness measurement in semiconductor processing. In cases where the film under examination is transparent to the illuminating radiation, ellipsometry can measure films down to one monolayer thick (3-10 Angstroms). However, ellipsometry fails in cases where the film under examination is opaque. Metallic films, which play a major role in integrated circuit fabrication, fall into this category. Optical radiation is absorbed within the first few tens to hundreds of Angstroms of the film, depending on the wavelength and material under examination. For example, using green radiation at a wavelength of 0.5 micron in aluminum, the absorption length is less than 70 Angstroms. At longer wavelengths, and in particular at infra-red wavelengths, this situation gets better, but still ellipsometry cannot provide the full solution with reference to metallic films or other optically opaque films.
Time resolved pulse-echo ultrasound is a well known technique for thickness measurement in situations where the thickness of interest is a few millimeters or at least tens of microns. For films used in semiconductor processing, one needs extremely short pulses so that the surface echo can be time resolved. Such pulses can be generated by short laser pulses and the general area of this art is known as photoacoustics. The physical processes involved is as follows: a short laser pulse is absorbed within one absorption length from the surface, causing a rise in local temperature of the surface. Through the temperature coefficient of expansion (expansivity) the film undergoes thermal stresses leading to an elastic pulse which propagates across the film at the speed of sound. Given the velocity of sound in the film, if one measures the time of flight across the film, one can compute the film thickness. The key-remaining issue, is therefore, the detection of the acoustic disturbance once it bounces back from the rear side of the film and reaches the front surface.
Reference is made to the work of investigators at the Brown University in U.S. Pat. No. 4,710,030. The patent states that once a stress pulse is reflected from the rear side of the film and reaches the surface, it changes the optical constants of the surface and near surface. It can be shown that these changes can be as low as a few parts in 106, depending on both the elastic and electronic properties of the film. The change in the optical constants of the surface leads to a change in reflectivity which is detected by monitoring the intensity in a xe2x80x9cprobexe2x80x9d beam which also illuminates the surface. Given that the change in optical constants is small, the method patented by workers at Brown University, at best, lacks sensitivity.
The above-described stress pulse not only changes the optical constants of the surface, but also causes a small displacement of the surface. Heterodyne interferometers have been used for detecting movement of surfaces, such as continuous ultrasonic displacements from rough surfaces. See, for example, xe2x80x9cHeterodyne Interferometric Laser Probe to Measure Continuous Ultrasonic Displacements,xe2x80x9d by Monchalin, Rev. Sci. Instrum., Vol. 56, No. 4, April 1985, pp. 543-546. In traditional heterodyne interferometric systems such as that described by Monchalyn, continuous waves (CW) laser sources are used. In the traditional heterodyne interferometer, such as the one described in the Monchalin article, the reference beam and the probe beam travel along different signal paths. Environmental disturbance, such as air turbulence and mechanical vibrations, may introduce different fluctuations in the two paths, causing random noise. Because of such random noise, the signal-to-noise ratio of traditional heterodyne interferometers is insufficient for measuring small phase differences, such as those encountered in semiconductor film thickness measurements. Another example of the traditional heterodyne interferometer is that described in U.S. Pat. No. 4,619,529.
It is, therefore, desirable to provide improved techniques for film thickness measurements.
As noted above, the key issue in measuring film thickness in semiconductors is the detection of the acoustic echo. Applicants propose instead the provision of a pair of probe pulse and reference pulse radiation that are substantially in phase with each other for measuring the acoustic echo. The probe pulse is directed to a circuit area of the sample when it is influenced by the elastic pulse created by the pump pulse and the reference pulse is directed to the same or a different surface area of the sample so that the pair of pulses are modified by the sample. The modified pulses interfere at the detector. At least one of the pair of pulses is modulated in phase or frequency before or after modification by the sample and prior to detection by the detector. By analyzing the detector output, film thickness information may be derived. Since reference and probe pulses are used for measuring the sample surface, the system proposed by Applicants has the required resolution to detect the acoustic echo caused by the elastic wave.
In the preferred embodiment, an optical delay may be used to alter a time relationship between the pump pulse and the probe pulse so that the probe pulse is directed to the sample surface when it is influenced by the elastic pulse created by the pump pulse.
Where the reference and probe signals in the interferometer do not travel along the same path, random noise created by environmental factors may render the interferometer impractical for measuring small phase changes caused by the acoustic echo at the sample surface. Therefore, preferably, the reference and probe pulses are directed along substantially a common optical path between an optical source and the detector.
Where measurement of very thin films or layers is desired, the probe and reference pulses used may have durations of less than about 10 picoseconds. Such scheme is applicable in both heterodyne and homodyne systems, so that one, or both or none of the two pulses is modulated in intensity or phase or frequency.